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Discussion Starter #1
Hello evryone,


I'm having an issue with my TPS. While I was riding the bike threw a C40 code (ISC Valve). While checking the code I noticed the TPS is responding very slow whenever I release the throttle. Now it's in the right position (c-00) on idle and raises to the top position once the throttle is twisted. But once I fully release the throttle it takes 4 to 5 seconds for the line to go back to the idle position. I know this isn't right but can't figure out why it's taking so long. Now my bike has ASV levers, Brembo clutch master cylinder and a Brembo micro switch as the clutch switch. I'm thinking my micro switch may have gone bad so I ordered another one. I'm hoping that's what's causing my C40 code. Anyone have an idea what would cause the TPS to respond so slow when I come out of the throttle. Thanks for your time!!!
 

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My K6 thankfully doesn't have an ISC so this is best guess. The TPS position display goes through an averaging process but it isn't that slow. I suspect that the slow response that you're seeing is due to the ISC adjusting the idle speed and this is affecting the TPS position display. Fix the ISC, then check again. Alternately check the idle speed as you release the throttle. But that will probably require that the bike be on a dyno where things can be logged.
 

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Wouldn't the slow response be entirely due to the smoothing you mentioned? I don't see how and why the ISC would affect the TPS. It regulates idle air through a separate passage, bypassing the throttle plates, doesn't it? In any case the behavior described in the OP, sounds natural to me and is what happens on my K4 750 as well. I'm not positive it takes 4-5s to respond, but it does take several seconds.
 

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Discussion Starter #4
I’ve checked my tps in the past and it’s never taken that long to return to idle once the throttle has been released. I’ve contacted my local Suzuki dealer and they said something isn’t right. I’m not a fan of bringing my bike to the dealer for anything. I prefer to work on my bike. But this has me stumbled. I’m hoping once the new brembo micro switch is installed this will cure the issue. But I’m not 100% certain it will. The switch should be in today. So I’ll wire it up and let you guys know the results. Thanks for looking and the advise it’s appreciated
 

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The TPS averaging is about .4 second per the manual. So something else seems to be going on. Furthermore the K5/K6 manual dances around it but it apparently wants the TPS voltage to be 1.125 when the bike is idling at 1150 RPM. So if you've got the idle speed correctly adjusted, you could adjust the TPS without the engine running by either the -c00 display or a voltmeter on the TPS. It may not mean a hoot but I'm betting that adjustment by voltmeter would be more precise. However the fast idle must not be engaged or it will mess everything up and that's probably why it isn't mentioned (temporarily unplugging the STVA connector ought to suffice). I think that something similar is true of K7 except that you've got the ISC in the mix controlling the idle (and fast idle). My thinking, which may be totally bogus, is that instead of a fixed target voltage, the ECM is altering the target based on things like the current idle speed and the ISC setting. This target altering could take place at a slower pace and be what he's seeing.

But as said, this is just a guess from someone who happily doesn't have an ISC.

Note that early on, people with K7/K8's were experiencing mysterious idle behavior that was traced to them disabling the clutch switch and its impact on the ISC. I don't remember C40 being mentioned, just the bike dieing instead of idling. The switch must be functional. But a Brembo switch ought to be fairly bulletproof unless he's been beating on it. Should be easy to check with an ohmmeter.

P.S. For the few here with aftermarket dashes (AiM, Translogic, etc.) there might not be a -c00 display and the above described voltmeter method may be the only way to set the TPS.
 

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Discussion Starter #6
Well the new switch came in so I wired it up and installed it. I tested both switches prior to installing the new one. Both had the same good continuity results and both had .3 ohms. I haven’t taken it out for a ride yet due to the rain. Hopefully tomorrow I’ll have time. I’m starting to think my ISC is at the beginning stage of going bad. Although I was hoping the clutch switch was the issue. Specially since the bike would stall upon deacceleration while riding. Which was one of the symptoms of the clutch switch.
 

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You might try removing the ISC and checking for carbon deposits. See 5-25 in the service manual. That stuff about the O-ring is some Suzuki baloney and, as best I can tell, it's available.

I'm assuming that you have had the ISC recall. There are markings on the recall part that identify it as such.
 

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Discussion Starter #8
After doing more testing looks like it’s my ISC valve. I bought the bike new in 07 and believe the dealer did perform the recall but I’ll double check with my local dealer on Monday. If it was already done I be ordering a new one and I’ll post the results. Thanks for all the help guys!
 

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The original ISC was 18117-10G00. It was replaced by 18117-21H10, which was identified by a yellow dot on the top. The ECM was also replaced to 32920-21H60 (E3). The current service part is 18117-21H11. It seems to have the same mark. But it runs around $190. For that much I'd first see if something like carb cleaner would get rid of deposits or otherwise help.



Note that the ISC is an emissions related device. You may be too late but there's a chance that it has an extended warranty - if it's the problem.
 

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Discussion Starter #10
After doing more testing looks like it’s my ISC valve. I bought the bike new in 07 and believe the dealer did perform the recall but I’ll double check with my local dealer on Monday. If it wasn't already done I’ll be ordering a new one and I’ll post the results. Thanks for all the help guys!
I’ll check for the warranty as well... thanks for that info!!!
 

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The manual does indeed state 0.4s per update on the LCD and that the dash should be stable for a at leas two updates (i.e. 1s) before it can be considered accurate. Funny, I have only adjusted my TPS once, years ago, but I remember that it took at least a second, or two to respond. So, do ISC-ecquipped models still have linkages between the primary and secondary throttle shafts, to control fast-idle via the STVA, like the older models? I would have expected, the ISC valve to be used to implement fast-idle.
 

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Linkage between the primary and secondary shaft is pushing it a bit IMO. The K5/K6 had a cam on the secondary throttle that the ECM would use to bump the primary throttle for fast idle. You can watch it function when the bike is cold started. I've forgotten but I think K2-K4 was similar in concept, but with a more complicated linkage, while K1 had a manual fast idle via a cable. I thought that the cam was gone in K7 and all idle adjustment, fast or otherwise, was done by the ISC under command of the ECM. But, surprisingly, the cam is visible on 5-14 of the service manual. However, unlike the earlier manual, there is no information on the fast idle or how to set it. The only references that I could find are on 4-24 and 10-12 and they seem to just be overlooked carryovers from the K5/K6 manual. AFAIK tweaking is only possible by the SDS.
 

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Perhaps the secondary shaft does bump the primaries open a small fixed amount and then further fast idle adjustment is done via the ISC? That would make sense, in allowing properly regulated fast idle, without needing an ISC valve with adequate range, to accommodate fast idling on its own. If that is the case, then the secondaries might indeed be causing the sluggish TPS response, but if they are, it should be easily visible through the cam/linkage on the left side of the throttle body.
 

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I see from 1D-8 and 1D-11 of the K9-L1 service manual that they have the fast idle cam & associated linkage of earlier years. But I don't see any mention of its function and on 1D-13, they say don't adjust the normal & fast idle screws (they don't refer to them as that). So I don't know what function that stuff has on those years. Fast idle is only mentioned once - on a generic troubleshooting form. We need someone to watch the cam during a cold start. An SDS log might be enlightening.

Here's an SDS log for a cold start from the K5/K6 service manual. I tried to insert it directly but this site wouldn't let me.

The engine was started at the 20 second mark. You can see the secondary throttle was initially fully open. At 35 seconds the secondary starts closing until it's 50% open at 45 seconds, at which point it fully closes. The primary throttle was bumped a bit by the action of the cam. That bump is the fast idle "setting" and it's gone by 40 seconds, at which point the idle drops from around 1500 to 1160 RPM. The closing ramp of the secondary is very typical and a giveaway that the fast idle is under ECM control. I'd like to see such a log for a K7-up

I initially went to ebay for throttle body pics. Don't know why but there are many K5/K6 throttle body listings but almost none for later years. Maybe the K5/K6's are all crashing from broken frames.
 

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Discussion Starter #16
The repair manual mentions using the sds tool to clear the faulty code. But not sure about needing to reset the ISC Valve
 

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Don't know. There's a "learned value" and a "pre-set" position mentioned on 5-26. There's also a "Spec" and a "Desired" idle speed mentioned on 4-81, both of which are different than "Engine Speed". I'm aware of this but have never had to deal with it. This was the first use of the ISC and the manual may not be as clear as it could be. You might try looking at the same sections in the K9-L1 manual to see if it's been improved. Unfortunately the later manual's layout is quite different.
 

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Discussion Starter #18
Ok so I installed the new ISC Valve and haven’t gotten a C40 code in 170 miles. I was hoping the new valve would also help the tps respond better but that’s not the case. So I ordered a new tps and should be in this week. I’ll update once he part is installed. Thanks for all the help.
 

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Does the TPS also fail to respond quickly with the engine not running? Note that, if our assumptions/conjectures above are correct, with the engine not running, it is possible that the TPS should read high when properly set.
 

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Discussion Starter #20
With the engine is off there’s also a delay. If the delay wasn’t 4 to 5 seconds long I wouldn’t be concerned. But my thinking of when the throttle is twisted. It pulls on the cables that move the linkage that runs through the throttle bodies and connects to the tps. The tps then sends that info back to the ecu. The ecu then tells the injectors more fuel is needed. So with that in mind when I’m completely off the throttle my injectors may be sending fuel into the cylinders when it’s not needed. Which could be the reason my plugs were fouled... just my thoughts...
 
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